1. Field of the Invention
The present invention relates to an antenna apparatus and a portable radio communication apparatus, and more particularly, is suitably applied to, for example, a compact portable radio communication apparatus.
2. Description of the Related Art
Conventionally, as shown in FIGS. 1A and 1B, a portable radio communication apparatus 1 in a digital portable telephone system of Personal Digital Cellular (PDC) system is configured to implement diversity reception with, for example, a whip antenna 2 and a plate-form inverted-F antenna 3, and thereby influence of phasing is reduced.
The whip antenna 2 is a line antenna being used as an antenna for transmission-reception installed from an upper surface of a box 4 approximately perpendicularly thereto, and is ordinarily selected to have length of around 1/4 wavelength to 1/2 wavelength. In addition, the whip antenna 2, which is brought into connection with a feed section 2A, is drawn out of interior of a box 4 at the time of communication (FIG. 1A), and is housed inside the box 4 when it is being carried (FIG. 1B).
The plate-form inverted-F antenna 3 is configured, as shown in FIG. 2, by comprising a rectangular radiation conductor 3A with a circumference length (L1.times.2+L2.times.2) being approximately 1/2 wavelength, a short-circuit section 3B to be short-circuited from one end of the above described radiation conductor 3A to a ground soil plate 5, and a feed pin 3D to bring a feed section 3C (FIGS. 1A and 1B) and the radiation conductor 3A into connection, and is ordinarily used as an antenna only for reception under a state that is built in the box 4.
In these whip antennas 2 as well as plate-form inverted-F antenna 3, transmission performance and reception performance are in reversible relationship, and unless otherwise notified later, transmission will be described, but reception will present similar characteristics.
Incidentally, in a thus configured portable radio communication apparatus 1 a whip antenna 2 is installed perpendicularly to the ground, and under this state, is arranged to oscillate radio wave of vertically polarized wave when transmission is implemented. On the other hand, an antenna in a cell station that communicates with the portable radio communication apparatus 1 also mainly utilizes vertically polarized wave so that the most excellent antenna characteristics will become available when polarized wave of both the parties correspond.
That is, as shown in FIG. 3, in the case where a portable radio communication apparatus 1 is used under a state that it stands straight up, polarized waves correspond so that good communication is implemented with respect to a cell station 7, but in the case where the portable radio communication apparatus 1 is applied to a user's ear under a state of inclining by approximately 60 degrees for use as at the time of communication, polarized waves do not correspond, giving rise to a problem that good communication is not implemented with respect to the cell station 7 due to deteriorated antenna characteristics.
Incidentally, as a method to cause polarized wave to correspond with polarized wave from the cell station 7 when the portable radio communication apparatus 1 at the time of communication remains under a state of inclining by approximately 60 degrees, it is considered that a whip antenna 2 should be caused to incline, but in this case, housing structure at the time when the apparatus is being carried gets complicated and does not provide good appearance in the visual standpoint for practical use.
In addition, as for the portable radio communication apparatus 1 (FIGS. 1A and 1B), feeding to the whip antenna 2 is implemented by a feed section 2A provided in an upper end section of the box 4, and therefore high frequency current flows out not only to a line antenna of the whip antenna 2 but also to the ground soil plate 5, and consequently, radio wave will be divided for radiation to the line antenna portion and the ground soil plate 5.
Actually, as shown in FIGS. 4A and 4B, the portable radio communication apparatus 1 will bring about current distribution shown by broken lines in the case where the whip antenna 2 is selected to have 1/4 wavelength or 3/8 wavelength so that radio wave is divided for distribution to the line antenna portion and the ground soil plate 5 and radiated.
Accordingly, with the portable radio communication apparatus 1, the ground soil plate 5 comes closer to human head section than the line form portion of the whip antenna 2 at the time of communication, and therefore radio wave to be radiated by leak current having flown out to the ground soil plate 5 will be strongly affected by human body, and consequently, antenna characteristics used to be deteriorated.
In addition, in order to prevent current from flowing out to the ground soil plate 5, it is considered that the line form portion of the whip antenna 2 is selected to have length of 1/2 wavelength, giving rise to, however, in this case, current distribution as shown in FIG. 4C so that any leak current can be prevented from flowing to the ground soil plate 5, nevertheless, the line antenna portion will become too much long.
Moreover, in the portable radio communication apparatus 1, as shown in FIG. 5, a plate-form inverted-F antenna 3 is installed in the position very near the ground soil plate 5, and therefore, current 5i1 flowing into the plate-form inverted-F antenna 3 will induce leak current 5i2 in the ground soil plate 5 that will flow in the vertical direction in large quantity, and thereby radio wave of vertically polarized wave is radiated dominantly.
Thereby, in the case where the portable radio communication apparatus 1, as at the time of communication, is used under a state of inclining by approximately 60 degrees, as with the above described whip antenna 2, polarized waves do not correspond with polarized waves from the cell station, giving rise to such problems that antenna characteristics are deteriorated, and at the same time radio wave radiated by leak current flowing out to the ground soil plate 5 is strongly influenced by a human body to deteriorate antenna characteristics.
Incidentally, as a method to attain excellent diversity effects, space diversity utilizing difference in installation point of antenna, angle diversity utilizing difference in directivity of antenna, and polarization diversity utilizing difference in polarization of antenna are generally well known.
However, the portable radio communication apparatus 1 has vertically polarized wave as its main polarized wave both for the whip antenna 2 and the plate-form inverted-F antenna 3, making effects due to polarization diversity hardly expectable. In addition, the portable radio communication apparatus 1 undergoes miniaturization, and in turn effects of space diversity drop while, as for effects of directional diversity, difficulty in giving any directivity with small antenna brings about such a problem that certain diversity effects remain unavailable.
Moreover, the portable radio communication apparatus 1 suffers from such a problem that leak current 5i2 (FIG. 5) in the vertical direction induced when current 5i1 is caused to flow to the plate-form inverted-F antenna 3 gets together with leak current flowing out to the ground soil plate 5 when a current is caused to flow into the whip antenna 2 to influence each other to deteriorate the antenna characteristics.